Consonant with a trend toward small form factor hard disk drives, linear voice coil actuator structures have become supplanted with rotary or swing arm voice coil actuators. Such actuators are used to position a stack of heads relative to circular data tracks formed on a stack of rotating disks. Conventionally, rotary actuators have included a cast aluminum alloy E-shaped block forming a bearing hub and a voice coil. The voice coil has been attached to the hub in some instances with an adhesive, and in other instances with overmolding of a plastic material. A head-gimbal assembly including a load beam carrying a gimbal and slider-transducer ("head") has been attached to an outer end of the E block. The load beam provides a preload force to the head to urge it against an adjacent disk data storage surface. As the disk rotates, the head overcomes the preload force and "flies" several microinches above the disk surface on an air cushion or bearing, in accordance with what has come to be known in the art as Winchester technology. The voice coil was positioned in a gap having an intense magnetic force field provided by one or more permanent magnets. Current passing through the voice coil caused the actuator rotor to rotate in one direction or the other, depending upon direction of current flow, in accordance with Maxwell's fight hand rule.
One of the drawbacks of the conventional die cast E-block was the cost incurred in die casting and precision machining the irregularly shaped E block to reach its required size tolerances. Another one of the drawbacks of the die cast E-block was its mass moment and relative high inertia. This high level of inertia has required more and more powerful voice coil motors, which have been achieved at greater cost by employing rare earth magnets, such as neodymium-iron-boron alloy permanent magnets for example. Another drawback with die cast E-blocks was transmission of unwanted acoustic noise energy to the drive housing due to inherently low internal damping of the die cast metallic structure
There are also drawbacks associated with all-plastic actuator hubs. One drawback of one published approach, found in U.S. Pat. No. 5,382,851 for "Swing-Type Actuator", is that the sheet metal head arms embedded in the molded plastic hub are not in direct, low-Ohmic electrical contact with the bearing assembly and the disk drive base. Failure to ground the head arms may give rise to electrostatic discharge and resultant damage to preamplifier ICs, thin film head structures, and particularly magneto-resistive read-transducers which are now coming into more widespread usage in hard disk drive technology.
While all-plastic E blocks have been variously proposed over the years and are present in the prior art, they have met with very little success or acceptance. Head attachment issues, thermal stability issues, electrical grounding issues, moving single-turn conductive plastic overmold, and vertical stiffness issues have remained substantially unsolved. Also, the inherent anisotropic properties of plastics, their tendency to creep or cold-flow over time and temperature gradients, and the shrinkage inherent to the plastic molding process have contributed to warpage, changes in coplanarity between the head stack and the rotating disks, and misalignment of the actuator support bearings.
Thus, a hitherto unsolved need has remained for a low cost plastic overmolded rotary voice coil actuator rotor which overcomes limitations and drawbacks of the prior art.